1. Field of the Invention
The present invention relates to power drive systems for or incorporating variable reluctance electrical machines and to variable reluctance electrical machines for such systems. More particularly, the invention relates to power drive systems for doubly-salient variable or switched reluctance motors and to reluctance motors of this kind for such power drive systems. The present invention also relates to a construction of variable reluctance machine operable as a generator.
2. Description of the Prior Art
Variable reluctance motors are among the oldest of electrodynamic machines, but their industrial application was for many years inhibited by the lack of availability of suitable switching means for reliable progressive sequential energisation of the stator poles to bring about rotation of the rotor. The lengthy history of relative lack of success in adapting variable reluctance electrical machines for use for higher power drives is emphasized in the discussion to the papers presented to the IEE Power Division Professional Groups P1 and P6 on Dec. 8, 1980 PROC., Vol. 128, Pt. B, No. 5, SEPTEMBER 1980, pp 260-268, where reference was made to the earliest such motors, designed in 1842 for railway use and demonstrably ancestors of today's machines. Reference was also made to a subsequent machine of 1851. Several contributors connected on the curious circumstances that machines of this kind should for so long have failed to find a commercial role, and much of the discussion revolved around the difficulty of successfully applying reluctance motors to an everyday industrial role.
Even the undoubted advances discussed and described at that Conference do not, however, appear to have brought about widespread use of variable reluctance motors in substitution for conventional industrial AC and DC units.
While variable reluctance motors have been used commercially in more recent times, in the form of stepping motors, the stepping motor is fundamentally a digital device controlled by pulsed inputs which yield predetermined output steps. The development of microprocessor control systems has enhanced the utility of stepping motor drives, but nonetheless these motors essentially remain suited to positioning applications and are not generally suitable for delivering significant power outputs. However the inherently high efficiency of the reluctance motor has caused the increasing availability of high power semiconductor switching devices in recent years to lead to increasing interest in the possibility of applying variable reluctance motors to higher power drives in industrial applications, while attention has also been drawn to the advantages of operating variable reluctance motors in the saturated mode, in which mode the reluctance motor is especially efficient in converting electrical energy into mechanical work.
Torque is generated in a reluctance motor when a rotor pole moves relative to a stator pole from a position of maximum reluctance into a pole overlap configuration in which the reluctance is a minimum. In a practical construction, a variable reluctance motor typically has a number of paired rotor poles and a greater number of paired stator poles. Thus there is a plurality of possible stable minimum reluctance positions, in each of which one pair of rotor poles is aligned with one pair of stator poles. Each stator pole pair is provided with energizing windings and when a particular pair is energized, a corresponding pair of rotor poles will move into alignment with those stator poles, thereby developing torque. If energization is then switched from that pair of stator windings to an appropriate other pair, the rotor may then be rotated further through an angle determined by the relationship between the numbers of rotor and stator poles to a new stable minimum reluctance position, and so on by further sequential energization. In particular when intended for a stepping drive, the machine may incorporate permanent magnets so that a force tending to hold the rotor in a specific displacement relative to the stator exists, even in the absence of energizing currents. Alternatively the stator poles may be magnetized only when exciting currents are present. In the absence of permanent magnets, currents in the reluctance motor are unipolar, i.e. they only flow through the windings in one direction, and the rotational direction of the machine is reversed by changing the order in which the windings are energized during each revolution of the rotor, rather than by reversing the direction of current flow through these windings. Accordingly for one direction of rotation, the stator pole windings are energized so that the rotor poles move into alignment with appropriate stator poles from one circumferential side of the stator poles. For the other direction of rotation, the sequence of stator winding excitation is such that the rotor poles move into alignment with stator poles from the other circumferential side of the stator pole.
In a machine with a multiplicity of poles therefore, the rotor may rotate to bring a pair of rotor poles into a configuration of minimum reluctance with a particular pair of stator poles from either of two directions, so that each pair of rotor poles has two possible positions of maximum reluctance relative to a particular stator pole pair, one such position being to one circumferential side of that pole pair and the other maximum reluctance position lying to the other circumferential side of the stator poles in question. Accordingly for a particular direction of rotation, the sequence of energization of the stator pole windings is that which will induce rotation of the rotor in the desired direction to bring rotor poles into a minimum reluctance relationship with stator pole pairs from the appropriate circumferential side of the stator poles. Furthermore since each stator pole winding pair or phase may be energized to bring about either forward or reverse rotation, thereby also developing either forward or reverse torque at the motor drive shaft, when therefore the windings are undergoing sequential energization to produce rotation in a selected direction, they should not be energized to any significant extent during the periods while rotor poles are moving away from their minimum reluctance dispositions in alignment with stator poles towards their maximum reluctance dispositions in relation to these stator poles, from which their displacement towards these poles begins for rotation in the opposite direction. Energization at this time will develop an opposing torque, acting against the torque now being developed during the continuing rotation of the rotor by the movement of a further pair of rotor poles into a minimum reluctance configuration with the next pair of stator poles now being energized in due turn for this continuing rotation. Thus each electrical cycle for each stator pole winding phase, i.e. typically a pair of stator pole windings, is distinguished by a half-cycle during which the phase is energized to produce torque to rotate the rotor in the selected direction, i.e. forward or reverse, and a further half-cycle during which the phase windings remain de-energized so that substantially no torque is developed which would tend to oppose the desired direction of rotor rotation. Reversal of the direction of rotation of the rotor involves therefore interchange of the energized and quiescent periods of the electrical cycle for each motor phase.
A variable reluctance motor may have typically three or four phases and during the period of excitation of each phase, one or more pairs of stator pole windings are energized for the appropriate half-cycle. The torque developed during the movement of a particular pair of rotor poles relative to an appropriate pair of stator poles may be plotted experimentally against the rotor angle, while the stator pole windings are energized with a DC current, to produce a so-called static torque/rotor angle characteristic. The phase torque output of the machine during operation may then be derived by plotting torque against rotor angle for the specific value of current with which each phase is energized at each angular position of the rotor. When the phases are energized with constant currents in an on-off manner, as is conventional in stepping motor practice, the phase torque output of the machine will have essentially the same shape as the static torque characteristic for each phase for the half-cycle appropriate to the desired direction of rotation. By suitable design of the machine in terms of rotor and stator dimensions, the start of the torque-producing region or half-cycle of each incoming phase may be arranged to overlap that of the outgoing preceding phase so that there is continuity of torque throughout the rotation of the motor by virtue of this phase torque overlap. Net output torque at the shaft is then computed by adding the phase torques. Depending on the precise shapes and angular extents of the phase torques, this net torque may exhibit a significant ripple during torque transition between phases.
In the application of known stepping motor systems to variable speed drives, it has been found that torque ripple during phase to phase transitions, is significant and may be such as to render the motor unacceptable for such drives. In such systems, the static torque against rotor angle characteristic for a single phase of a saturable variable reluctance motor during a rotor displacement from a maximum reluctance position to a minimum reluctance position is typically distinguished by a very rapid initial rise in torque as pole overlap commences, followed by a period during which torque remains substantially constant while pole overlap progresses towards full overlap, and the characteristic terminates with a roll-off portion during which torque drops significantly as full overlap is achieved and the relevant rotor pole moves into a disposition of minimum reluctance. Further displacement of the rotor relative to the stator then leads to the poles moving out of overlap, and the static torque characteristic of this displacement is substantially an inverse mirror image, about the zero-torque full overlap condition, of that for the displacement into the overlap condition, the direction in which the torque is exerted being reversed. This negative torque developed by the further relative displacement of the rotor and stator poles from their minimum reluctance relationship terminates with arrival of the rotor in a new position of maximum reluctance, from which a further complete cycle may commence, with displacement of the rotor pole into overlap with a further stator pole taking place. While the magnitude of the peak static torque will vary depending on the level of energizing current, the general shape of this characteristic remains the same for all levels of excitation. Accordingly regardless of the extent of the overlap between successive phase torques and the levels of the exciting currents, each incoming phase torque-generating half-cycle has a region during which torque rises very rapidly and typically much more rapidly than the rate at which torque produced by the torque-generating half-cycle of the outgoing phase decays, so that the net machine torque is not smooth and the phase to phase torque transfers are distinguished by substantial torque fluctuations or ripple.
Apart from its deleterious effect on torque smoothness during transition between phases, this rapid torque rise experienced in many known reluctance motors at the start of pole overlap, especially when the windings are energized with constant or stepform energizing currents, also frequently leads to generation of vibration and noise in operation of the motor. The rapidly rising force at the start of the torque/angle characteristic has the same effect as an impulsive "hammer-type" blow. Structural resonance in the motor may be triggered by the repeated torque impulses, leading to inter alia stator bell mode vibration in which the inward attraction of diametrically opposite stator poles produces an electrical deformation of the stator. As this deformation progresses around the stator, a bell-like resonance is produced. Other modes of resonance may include a rotor radial mode arising from deflection or distortion of the rotor under the electrical forces, bearing rattle which may arise out of any looseness in the fit of the bearings on the rotor shaft, and a torsional mode excited by the rotation-inducing torsional forces acting on the rotor. Any or all of these modes of resonance may be present and result in noise and vibration. While they may be damped by such known methods as the use of heavier bearings and structures than are required by electromagnetic considerations alone, such a solution is not fundamentally a satisfactory answer to the vibration and resonance problems frequently experienced in these machines.
The achievement of both torque smoothness and freedom from noise and vibration in operation is dependent on the complex static-torque versus rotor angle characteristic of the variable reluctance motor but torque smoothness and freedom from noise and vibration are not necessarily cured by the same remedies, and in particular a motor in which the characteristics are such that the torque ripple at phase torque transitions is perhaps acceptable for certain drive purposes may not necessarily be distinguished by silent and vibration-free operation.
Variable reluctance machines for use in power drives have been described in U.S. Pat. Nos. 3,062,979 and 3,171,049 of Jarret and U.S. Pat. No. 3,956,678 of Byrne and Lacy. In U.S. Pat. No. 3,062,979 of Jarret, the saturation induction in the rotor teeth magnetic material of a variable reluctance electric machine is reduced to between 15 and 85% of the maximal induction selected for the magnetic circuit material of the machine, with the purpose of allowing magnetic fields of relatively large strength to be developed in the gaps adjacent the polar areas without excessive losses and to promote a high ratio of output power of the machine to its weight. In order to achieve this object, the rotor teeth are constituted by alternate sheets of magnetic material interspersed with non-magnetic material such as lamina-shaped airgaps. Jarret's U.S. Pat. No. 3,171,049 describes a development of the machine of the earlier U.S. Pat. No. 3,062,979 in which the rotor is divided into two co-axial half-rotors axially spaced apart and secured to each other and to a common rotatable shaft. The stator is similarly divided and the windings are then connected in the form of a four-impedance bridge in order to achieve effective decoupling of the AC and DC circuits of the machine and thereby an improved level of machine efficiency. In another aspect the machine of this patent specification is shown to have a plurality of rotor teeth, each of which is defined by a number of sectoral fanned-out portions, so that the movement of the tooth past a stator pole is accompanied by a stepwise change in magnetic characteristic and the machine may be operated substantially with sine-wave current. The purpose of this arrangement is to alter the waveform of the induced voltage from the substantial rectangular shape induced by the movement of a rotor tooth past a stator pole in those constructions of machine in which the rotor teeth have constant magnetic properties throughout their angular development. In the particular constructions shown in U.S. Pat. No. 3,171,049, each tooth is defined by a plurality of groups of sectoral projections, the groups being spaced apart in the axial direction by air gaps and each tooth being defined by four laminations in a fanned array so that each step in the stepwise change of magnetic characteristic involves a stepform incremental increase or decrease of one fifth, as each tooth lamination comes beneath or moves away from a stator pole. In U.S. Pat. No. 3,956,678, stepping motor techniques are described in which simplification of the drive and a gain in specific output and efficiency are achieved by constructions ensuring maximum saturation of magnetic flux at the stator pole faces, the airgaps between associated stator and rotor pole faces being minimal. A rotor pole structure is described in which a leading part of each rotor pole surface is undermined by deep trapezoidal slots to reduce the gap flux density compared with the unslotted pole surface portions, the objective being to extend the mechanical displacement of the rotor over which a uniform rate of flux increase occurs to correspond to one stator pole pitch, thereby also providing torque continuity with a two-phase configuration.
None of these prior art documents contain any comprehensive consideration of the problems of torque ripple and noise generation discussed above. While the Jarret patent specifications disclose a number of features relating to reluctance motor torque in general, the alleged improvement in torque output achieved by reducing the packing factor of the laminated steel in the pole face in the arrangement of U.S. Pat. No. 3,062,979 appears to be based on a misconception, since the mean torque is in fact always reduced by such measures, although reducing the packing factor in the pole face does in some cases offer the possibility of altering the shape of the static torque against angle diagram at constant current with little if any significant loss of static mean torque. In an arrangement described by Byrne and Lacy in a paper entitled "Characteristics of Saturable Stepper and Reluctance Motors" delivered to the Small Machines Conference in 1976 (IEE Conf. Publ. 136, 1976 pp 93-96), an echelon array of rotor stampings of graded arc length is used, by virtue of which an improved degree of torque uniformity is achieved over the step length of 90.degree.. By virtue of the echelon array, the constriction cross-section in the overlap between stator and rotor increases linearly over the 90.degree. step length arc.
In the classis stepper motor such as is frequently used in computer peripherals and also to some extent in numerically controlled machine tools for positioning movements, the problems of torque transitions and torque ripple are not of major consequence, since the motor is essentially used only for digital positioning purposes in which its incremental operation or stepping advance between positions of minimum reluctance of the rotor is employed, and it is not called upon to develop significant levels of power. Thus, the exact shape of the static torque versus angle characteristic is not of major consequence, since the power levels in question are modest and the impulsive force generated by the initial sharp increase in torque as overlap commences is sufficiently small in absolute terms as not to be of great significance insofar as noise and vibration generation is concerned. In addition, it has been observed that in for, example, a 200-step permanent magnet stepping machine, in which the stator and rotor teeth are defined by semicircular cutouts in the stator pole faces and the rotor periphery respectively, these cutouts defining the gaps between the teeth, the successive torque versus angle characteristics of the phases tend to be smoothed and may attain an approximately sinusoidal shape. Since in a stepper motor of this kind, the number of teeth is large relative to the dimensions of the rotor and stator, the airgap between the rotor and stator is also relatively large in terms of the dimensions of the individual teeth, and it is believed that this may contribute to the smoothed shape of the torque-angle curve. In a technique known as microstepping, a stepper motor of this kind may be fed with individually controlled currents in its individual phases so as to produce null positions additional to those arising from the rotor positions of minimum reluctance. In one such technique, the phase currents are applied in the form of sine and cosine waves dependent on the angle of electrical phase displacement. The system uses a counter which accepts a series of input signals in the form of pulses and generates digital numbers representing the sine and cosine values, look-up tables in the form of ROMs being usable for this conversion. The resulting reference signals are used to control the current by means of a chopper drive.